The use of self-powered power sensors (SPPSs) placed at a plurality of locations within a facility's electrical grid allows for the measurement of power consumption throughout the grid. Typically, such SPPSs are positioned at points of interest of power consumption, such as at circuit breakers or at higher power consumption devices.
FIG. 1 is a block diagram of an exemplary SPPS 100 that includes a current transformer (CT) 103 having its primary winding accepting the primary current 102. It further includes an analog-to-digital converter (ADC or A/D) 105 that converts the analog signal into a series of digital samples under the control of a clock or timer 113 and a microcontroller (MC) 107. The samples gathered by the ADC 105 are processed by the MC 107 and then, using a transceiver 109, the processed information is transmitted using an antenna 116. An oscillator 110 oscillates at a desired frequency and feeds to the clock or timer 113.
FIG. 2 is a block diagram of a modified SPPS 200 that includes the same basic elements as described for the SPPS 100 depicted in FIG. 1. The modified SPPS 200 further includes a low pass filter (LPF) 202 coupled to the CT 103 and ADC 105. In one embodiment, LPF 202 is an analog filter. The input current 201 may be derived from the CT 103. The LPF 202 may be implemented in many different ways including, but not limited to, passive or active filters, matched filters, or harmonic filters. According to one embodiment, as shown in FIG. 2, the filtered signal 203 is fed to the ADC 105. The MC 107 can select what signal shall be sampled (i.e., an input signal 104 or a filtered signal 203). In other embodiments, the ADC 105 includes two channels, where both signals can be sampled simultaneously. According to one embodiment, the LPF 202 can be activated only when sampling the fundamental signal 203, thus saving power consumption.
FIG. 3 is a block diagram of yet another modified SPPS 300 that includes the same basic elements as described for the modified SPPS 200 depicted in FIG. 2. The SPPS 300 further includes a zero-crossing (or zero-cross) detector circuit 301 connected to the LPF 202 and the clock or timer 113. The filtered signal 203 is fed to a zero-crossing detection circuit 301 which generates a signal 302 relative to the zero-crossing time of the filtered signal 203. Further, according to this embodiment, the generated signal 302 may be time-stamped by, for example, the clock or timer 113. According to one embodiment, the zero-crossing detection circuit 301 may include a comparator, preferably with a small aperture to avoid significant errors in the zero-crossing detection.
Regardless of these and other embodiments for determining power consumption, the currently available SPPSs allow for only limited accuracy of measurement. That is, to accurately measure the power consumption of a load it is necessary to measure the power factor. Such a factor contains information of the phase difference between the voltage and the load current, as well as the distortion of the load current and voltage from a pure sinusoidal wave. In previous power measuring systems, voltage and current are simultaneously measured by the same physical device and therefore the relationship between the two signals is easy to calculate and measure. However, when there is a wireless connection between the current and voltage sampling, accurate measurement requires additional innovation to overcome the deficiencies of wireless SPPSs for measuring power consumption accurately.
It would therefore be advantageous to provide a solution that would overcome the challenges noted above.